1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus.
2. Description of the Related Art
An optical scanner is widely used for an image forming apparatus such as an optical printer, a digital copier, and an optical plotter. Recently, the optical scanner has been required to be manufactured at low cost, to be unaffected by environmental fluctuations, and to form an image with high-resolution.
When various lenses to be used in the optical scanner are made of a resin material, such a resin-made lens is light in weight, can be made at low cost, and have easiness in forming a particular surface shape typified by an aspherical surface. Therefore, an adoption of such particular surface shape in the resin-made lens enables enhancing optical properties and reducing the number of lenses included in an optical system.
In other words, the adoption of the resin-made lens contributes to reduction in size, weight, and cost, of the optical scanner. However, because a shape and a refractive index of the resin-made lens change due to the environmental fluctuations, especially due to temperature variation, the optical properties, especially a power, change from a designed value, and a “beam spot diameter” as a diameter of an optical spot on a scanned surface changes due to the environmental fluctuations.
Because, the power of the resin-made lens fluctuates in a positive lens and a negative lens inversely to each other, a method is often used in which “changes in the optical properties due to the environmental fluctuations” that occur in the positive and the negative resin-made lenses in the optical system of the optical scanner are mutually offset.
A semiconductor laser as a common optical source of the optical scanner has properties that an emission wavelength slants to a long-wavelength side upon rise in temperature (“change in a wavelength due to the temperature variation”), and that the wavelength changes due to “mode hop”. A change in the wavelength imposes a change in the properties due to chromatic aberration of the optical system used in the optical scanner, the change in the properties further causing a change in the beam spot diameter.
Therefore, in the optical scanner including the resin-made lens in the optical system and uses the semiconductor laser as the optical source, it is necessary to produce an optical design by considering the change in the optical properties due to the temperature variation and the change in the optical properties due to the change in the wavelength in the optical source.
For example, Japanese Patent Application Laid-Open No. 2005-258392 discloses an optical scanner (a laser scanner) which stabilizes the optical properties by adopting a diffracting surface in the optical system located at a stage before a deflector in the light passage (hereinafter referred to as “optical system before the deflector”) in consideration of the change in the optical properties due to the temperature variation and the change in the wavelength in the optical source. Furthermore, Japanese Patent Application Laid-Open No. H11-223784 discloses an optical scanner which employs the diffracting surface in the optical system located at a stage after the deflector in the light passage (hereinafter referred to as “optical system after the deflector”).
In Japanese Patent Application Laid-Open No. 2005-258392, even if an optical element having the diffracting surface in the optical system before the deflector, it is necessary when using the resin-made diffractive lens to consider, because of the following reasons, a returning light to the semiconductor laser, which is not referred to in the technology of Japanese Patent Application Laid-Open No. 2005-258392.    Reason 1: A reflected scattered light on the diffracting surface is larger than that on a refractive surface in quantity.    Reason 2: A resin has lower melting point and more difficulty in coating than a glass.
When such reflected scattered light again returns to the semiconductor laser, the following problems arise.    Problem 1: When the beam of the optical source enters again in a resonator of the semiconductor laser, an outgoing light quantity varies and density irregularities occur on an image.    Problem 2: The beam reflected again on an end facet or a stem of the semiconductor laser is returned to the scanned surface, and the density irregularities occur on the image.
There are many examples of a scanning optical system which has the diffracting surface, including Japanese Patent Application Laid-Open No. H11-223784. When the diffracting surface is employed in the scanning optical system, the beam deflected by the deflector is passed, and therefore it is not necessary to consider the returning light to a laser diode (LD). However, a difference occurs between the change in a magnification ratio of a main scanning due to the temperature variation which causes changes in the shape and the optical source wavelength at the same time, and the change in the magnification ratio of the main scanning only due to the change in the optical source wavelength such as skipping of the wavelength. As a result, when the optical scanner is loaded in a multi-color image forming apparatus, a significant color drift occurs.